ONTARIO, Canada (WCMH) — A new study says that men who have older brothers have an increased chance of being gay.

A study published in journal PNAS states that there is a biological explanation as to why men with older brothers have a greater chance of being gay.

CNN explains that according to the study, that when a woman gets pregnant with her first boy, a protein linked to the Y chromosome, which women do not have, gets into her bloodstream. Her body then recognizes the protein as a foreign substance, and her immune system responds, creating antibodies. If enough of these antibodies build up in the woman’s body and she gets pregnant with another a boy, they can cross the placental barrier and enter the brain of the second male fetus.

“That may alter the functions in the brain, changing the direction of how the male fetus may later develop their sense of attraction,” said study author Anthony Bogaert, a Canadian psychologist and professor in the departments of psychology and community health sciences at Brock University.

Previous studies have shown that the more older brothers a man has, the higher the chance he’ll be gay.

“Our study is a major advance in understanding the origins of sexual orientation in men by providing support for a theorized but previously unexamined biological mechanism—a maternal immune response to a protein important in male fetal brain development—and by beginning to explain one of the most reliable correlates of male homosexuality: older brothers,” the study concludes.